Migration is a process that attempts to reconstruct an image of an original reflecting structure from energy recorded an the surface in seismic traces.
Prior art migration processing techniques expend a great deal of effort to produce a snacked section from common mid-point (CMP) gathers, followed by a post-stack migration based on the stacking velocities. Stacking velocities however, require higher velocities for dipping events than those required for post stack migration, and some form of migration velocity estimation is required. Further advances in the prior art recognized that smearing from dip compensated velocities could be corrected by the inclusion of dip moveout (DMO) and prestack migration. The use of these prestack processes in velocity analysis loops enabled a more accurate estimate of the subsurface velocities and improved subsurface images.
DMO and post stack migration is currently more economical than conventional prestack migration, consequently he DMO method tends to be the current processing standard. However, the use of DMO is generally restricted to areas with smoothly varying velocities. In areas where the smooth velocity criteria fails, prestack migration is currently the preferred processing method. Typical prestack migration methods include migration of source (or shot) records (shown in Schultz, P. and Sherwood, 1980, Depth migration before sack, Geophysics, Vol. 45, pp. 376-393); migration of constant (or limited) offset sections (shown in Sattlegger, J. W., et al., 1980, Common offset plane migration (COPMIG), Geophys. Prosp., Vol. 28, pp. 859-871, Deregowski, S. M., 1990, Common-offset migrations and velocity analysis, First Break, Vol. 8, pp. 225-234); and migration by alternating downward continuation between shot and geophone (S-G) gathers (shown in Diet, J. P. et al., 1993, Velocity analysis with prestack time migration using the S-G method: A unified approach, Technical Program, Soc. Expl. Geophys. 63rd Ann. Int. Mtg., Washington, D.C., pp. 957-960).
Kirchhoff's method is a common prestack migration procedure that can be used in many of the above recited migration algorithms. Full prestack Kirchhoff migration is a stand-alone process described by summation of input samples directly to the output migrated sample (described in Yilmaz, O. 1987, Seismic data processing, SEG, pp. 331-334). The common use of full prestack migrations is limited by computer hardware long on run times.